It is no surprise siblings with past crimes carried out the attacks on Brussels

Khalid (left) and Ibrahim el-Bakraoui, the Belgian brothers identified as the suicide bombers who struck Brussels on 22 March. Belgian Police The news that two siblings - Khalid, 27, and Ibrahim el-Bakraoui, 30 - were responsible for the explosions at Brussels airport and a metro station on Tuesday should surprise no one. Nor should the news of their criminal records for violence.

Analysts have long been conscious of the high number of young men with previous involvement in petty or serious crime among volunteers and the importance of family ties in contemporary Islamic militancy.

They are increasingly aware that ISIS has combined the two in a devastatingly effective way.

A study of the 2001-2009 period, before the rise of ISIS, showed that around a quarter of militants identified in Europe had convictions. This proportion, evidence suggests, has risen. Mohammed Merah, who killed seven in France in 2012, was known for stealing cars.

Michael Adobawale, who stabbed a British soldier to death in 2013, was jailed after being caught up in a crack deal. In August last year, Belgian officials said that half of Belgian jihadis had a conviction, usually for theft and assault. Several of the Paris attackers in November had histories of petty crime, too.

The el-Bakraoui brothers appear to have been involved in more serious offences, however, such as armed robbery and gun dealing. This is rarer, but far from unknown.

Boys ride on their bicycles past shops in the neighbourhood of Molenbeek in Brussels, Belgium. Thomson Reuters

In Sweden, criminals have mixed human trafficking and participation in the Syrian civil war while Islamic extremists in France have been involved in armed criminal violence since the 1990s. That trend continues today - Amedy Coulibaly, who killed five people in Paris in January 2015, was radicalised while serving a six-year sentence for robbing a bank - and has contaminated violent activism in neighbouring Belgium.

Why keep it in the family? There are obvious advantages - for terrorism as for any other illegal or clandestine activity. Siblings are loyal and predisposed to help. They share language, values and attitudes, all imbued during a common childhood and youth.

We have seen acts of terrorism carried out by siblings before, as in the Boston Marathon bombing. Thomson Reuters

They are also, usually, nearby. Terrorism is a social activity, and its mechanics resemble any other social activity, despite its terrible, tragic consequences. Radicalisation is more about who you know, than what you know. Networks do what they can with what they've got to hand. Almost all attacks in Europe in recent years have involved local men attacking local targets with local materials. There is no one closer to home than a brother or, in rare cases, a sister.

The criminality of militants and these blood relationships combines in gangs, long seen by social scientists as a form of substitute family existing on, or often well beyond, the margins of legality.

In a recent report, Rik Coolsaet, a respected Belgian expert on terrorism, noted how much "street gang dynamics and ... foreign fighters' networks" had in common. This included recruitment, age range, the process through which individual members end up embracing the opinions of the majority, propensity for violence, and a sense of social alienation.

Ayman al’Zawahiri. Screen grab

ISIS has been able to exploit this in a way that al-Qaida, led by Ayman al'Zawahiri, an ascetic, bespectacled 64-year-old never could. The older group's videos usually involve a lengthy monologue in Arabic reminding Muslims of their various jihadi duties and little else. There is much emphasis on theological argument, aimed at those Muslims uncertain of the group's radical re-interpretation of core tenets of the faith.

This is absent from ISIS propaganda, which is spearheaded by short clips resembling a cross between a video game and a trailer for an action movie. These promise weapons, camaraderie and excitement. Publications such as the multilingual ISIS magazine Dabiq even emphasise sexual opportunity too, either through marriage to another volunteer or rape. The leader of ISIS, abu Bakr al'Baghdadi, is a generation younger than al'Zawahiri.

Becoming involved in ISIS is "a shift to another form of deviant behaviour, [which] .... opens a thrilling ... dimension to their way of life, transforming ... delinquents without a future into mujahideen with a cause, at least in their own view," Coolsaet wrote.

A fighter of the Islamic State of Iraq and the Levant (ISIL) holds an ISIL flag and a weapon on a street in the city of Mosul, June 23, 2014. REUTERS

In interviews in Brussels shortly after the Paris attacks, ISIS followers, the mother of a dead fighter, social workers in Molenbeek, often described as the jihadi centre, and one of the local security services' top specialists on Islamic militancy all reached a rare consensus on the attraction of ISIS. It was seen, they all said, as "the biggest street gang around".